A variety of types of chemical devices have been used in the detection of chemical processes. One type is a chemically-sensitive field effect transistor (chemFET). A chemFET includes a source and a drain separated by a channel region, and a chemically sensitive area coupled to the channel region. The operation of the chemFET is based on the modulation of channel conductance, caused by changes in charge at the sensitive area due to a chemical reaction occurring nearby. The modulation of the channel conductance changes the threshold voltage of the chemFET, which can be measured to detect or determine characteristics of the chemical reaction. The threshold voltage can, for example, be measured by applying appropriate bias voltages to the source and drain, and measuring a resulting current flowing through the chemFET. In another example, the threshold voltage can be measured by driving a known current through the chemFET, and measuring a resulting voltage at the source or drain.
An ion-sensitive field effect transistor (ISFET) is a type of chemFET that includes an ion-sensitive layer at the sensitive area. The presence of ions in an analyte solution alters the surface potential at the interface between the ion-sensitive layer and the analyte solution, for example, from the protonation or deprotonation of surface charge groups caused by the ions present in the analyte solution. The change in surface potential at the sensitive area of the ISFET affects the threshold voltage of the device, which can be measured to indicate the presence or concentration of ions within the solution. Arrays of ISFETs can be used for monitoring chemical reactions, such as DNA sequencing reactions, based on the detection of ions present, generated, or used during the reactions. More generally, large arrays of chemFETs or other types of chemical devices can be employed to detect and measure static or dynamic amounts or concentrations of a variety of analytes (e.g. hydrogen ions, other ions, compounds, etc.) in a variety of processes. The processes can, for example, be biological or chemical reactions, cell or tissue cultures or monitoring neural activity, nucleic acid sequencing, etc.
An issue that arises in the operation of large scale chemical device arrays is the susceptibility of the sensor output signals to noise. Specifically, the noise affects the accuracy of the downstream signal processing used to determine the characteristics of the chemical or biological process being detected by the sensors. It is therefore desirable to provide devices including low noise chemical devices, and methods for manufacturing such devices.
The use of the same reference symbols in different drawings indicates similar or identical items.